Estimating the Influence of Wind Load on Ice- and Snow-Covered Wires and Trees for Use in Impact-Based Indices

Abstract The combined hazards of high wind speeds and ice or snow accretions on trees or overhead wires can result in catastrophic damage. Here, a physically based accretion parameter is derived by 1) estimating the total wind-plus-accretion load upon a horizontally oriented cylinder and 2) computing an equivalent, thicker accretion value that would give the same magnitude of load under no wind conditions. This load-equivalent accretion value is dependent upon the assumed geometry of the accretions, with uniform radial thicknesses assumed in the case of ice or in the case of snow accreting upon a rotating structure or a uniform snow depth in a cap formation in the case of snow upon a rigid structure. If appropriate, wind gusts can be used instead of wind speeds in the parameter equations to account for structural loading from wind gusts. The new parameter scales continuously with both wind and raw accretion values and can be used directly within impact-based indices to highlight areas with anticipated impacts to trees or overhead utility lines. In direct comparison to other indices that are either discontinuous or do not account for wind influences, output using the parameter is continuous and aligns well with reported power outages and utility or tree damages for several cases herein. The parameter becomes incredibly powerful for probabilistic impact-based products, where the probability of the single variable is obtained directly versus estimating the joint probability of the two separate hazards (i.e., wind and winter accretions). Significance Statement Wind can worsen impacts and damage to trees and power lines during ice and snow storms. Accounting for the combined effect of wind and snow or ice accumulations in forecast products designed to highlight impact areas is crucial for accuracy. This paper derives equations to estimate the influence of wind as a simple variable that can be used within impact products. This concept is demonstrated with improved results over other methods that define impacts with two variables (e.g., wind and ice accumulation) or simply using accretion masses (and no wind influences). This work is particularly suited for probabilistic-based impact indices, where the probability of a single variable is easier than determining the joint probability of hazards (i.e., wind and accretions).